1. Field of the Invention
The present invention relates to a spool valve apparatus, which includes a spool that is urged in an axial direction by a compression coil spring.
2. Description of Related Art
Japanese Unexamined Patent publication No. 2002-243057 discloses a spool valve apparatus, which includes a spool valve and a drive means (e.g., an electromagnetic actuator) to change fluid (e.g., oil), adjust a flow quantity of the fluid or adjust a pressure of the fluid.
FIGS. 7 and 8 show an exemplary case where a previously proposed spool valve apparatus is used in an electromagnetic hydraulic pressure control valve.
Specifically, FIGS. 7 and 8 show the electromagnetic hydraulic pressure control valve, which adjusts a degree of communication between an input port 107 and an output port 108 and a degree of communication between the output port 108 and an effluent port 109 through adjustment of an axial position of a spool 104. The electromagnetic hydraulic pressure control valve includes a spool valve 101 and an electromagnetic actuator 102. The spool valve 101 includes a sleeve 103, the spool 104 and a compression coil spring 105. The electromagnetic actuator 102 drives the spool 104 of the spool valve 101.
More specifically, FIG. 7 shows a normally open (hereinafter referred to as “N/O”) type electromagnetic hydraulic pressure control valve, in which the degree of communication between the input port 107 and the output port 108 is maximized while the degree of communication between the output port 108 and the effluent port 109 is minimized (closed) in an OFF state of the electromagnetic actuator 102.
Furthermore, FIG. 8 shows a normally closed (hereinafter referred to as “N/C”) type electromagnetic hydraulic pressure control valve, in which the degree of communication between the input port 107 and the output port 108 is minimized (closed) while the degree of communication between the output port 108 and the effluent port 109 is maximized in the OFF state of the electromagnetic actuator 102.
The spool valve 101 adjusts the degree of communication of each port 107, 108, 109 by axially moving the spool 104 in the tubular sleeve 103. Thus, a slide clearance exists between the sleeve 103 and the spool 104 (specifically, each land 114, 115, 116). A portion of the hydraulic pressure leaks through the slide clearance, so that there is a demand for decreasing the amount of leakage of the hydraulic pressure through the slide clearance.
A seal performance for sealing the oil at the slide clearance is determined based on an axial distance (an axial seal distance) of the slide clearance and a radial size (a radial clearance size) of the slide clearance.
The axial distance of the slide clearance is limited by, for example, the installability of the spool valve 101, more specifically an available installation space of the spool valve 101.
In order to significantly reduce the radial size of the slide clearance, the processing accuracy for processing the sleeve 103 and the spool 104 needs to be increased, resulting in an increase in the manufacturing costs. Also, when the radial size of the slide clearance is significantly reduced, the slide resistance is increased, thereby deteriorating the responsibility of the spool 104. Furthermore, even when the radial size of the slide clearance is significantly reduced, the slide clearance still exits, and thereby the oil still leaks through the slide clearance.
In order to address the above disadvantage, it is conceivable to reduce an outer land diameter of each land 114, 115, 116 of the spool 104 to reduce an entire clearance cross sectional area of the slide clearance to limit the oil leakage of the spool valve 101 through the slide clearance.
In the previously proposed technique, as shown in FIGS. 7 and 8, the outer land diameter of each land 114, 115, 116 is larger than a spring diameter (a means spring diameter) B of the compression coil spring 105. Thus, the outer land diameter of the closest land (the effluent seal land 115 in FIG. 7 or the F/B land 116 in FIG. 8), which is closest to the compression coil spring 105, is larger than the spring diameter B of the compression coil spring 105 (A>B). Therefore, a spring support 200, which supports the compression coil spring 105, can be provided to the spool 104.
(1) However, when the spring diameter B is reduced in response to the reduction of the outer land diameter A, the spring performance of the compression coil spring 105 is significantly reduced, so that the technique of reducing the outer land diameter A is not acceptable in the cases of FIGS. 7 and 8.
(2) Furthermore, it is conceivable to reduce the spring diameter B of only an end portion of the compression coil spring 105, which contacts the spool 104 (see, for example, the technique of reducing the spring diameter of the spring recited in Japanese Unexamined Patent Publication No. 5-126275). However, similar to the above case, the end portion of the compression coil spring, which has the reduced spring diameter, cannot provide the sufficient spring performance. Therefore, the technique of reducing the outer land diameter A is still not acceptable.
(3) With reference to FIG. 9, there is also another technique, in which a projection 201 is provided at the axial end of the spool 104, and an annular plate-shaped (doughnut-shaped) spring seat 122 is installed by, for example, press fitting to the projection 201 to receive the compression coil spring 105. In this way, only the outer land diameter A can be reduced while the spring diameter B is unchanged.
However, the spool 104 needs to have a spring seat support surface 202 around the projection 201. Thus, when the outer land diameter A is reduced, it is difficult to provide the required spring seat support surface 202. Therefore, the spring seat 122 cannot be installed to the spool 104.
Because of the above reasons (1) to (3), in the previously proposed techniques, it is not possible to reduce the outer land diameter A of the closest land of the spool 104, which is closest to the compression coil spring 105 while maintaining the sufficiently large spring diameter B at the end portion of the compression coil spring 105, which contacts the spool 104.